1. Field of the Invention
The present invention relates to a passive- or active-matrix liquid crystal display device. Particularly, the present invention relates to an electrode structure of a transflective liquid crystal display device that possesses functional properties of both transparent and reflective liquid crystal display devices.
2. Description of the Related Art
In recent years, displays adaptable to changes in their use environments, power savings, and weight reductions have been desired for the explosive spread of handheld terminals, typified by cellular phones.
From the viewpoints of achieving the reduction in thickness and weight, a liquid crystal display (LCD) device or an organic electroluminescent (EL) display device may be of typically promising in the art.
A transparent liquid crystal display (LCD) device features its low power consumption only for actuating its display device. In this case, however, the liquid crystal itself does not emit light. Therefore, the liquid crystal needs backlighting when it is used in such a display device for providing information or graphics thereon. In uses for cellular phones, electroluminescent (EL) backlights are generally provided. However, such an EL backlight requires an additional amount of electric power. Therefore, it is difficult to take advantage of the characteristic power saving of the liquid crystal in a sufficient manner, so that the EL backlight has disadvantage for a reduction in power consumption. Under the dark environment, a good contrast display can be viewed on the screen of the display device. Under the normal lighting environment, however, such a display cannot be viewed sufficiently. Therefore, the adaptability to different use environments is insufficient in case of either an upper output system or a lower output system.
Furthermore, the organic EL display device is characterized in that a display element itself emits light. The power consumption of the organic EL display device is higher than that of the reflective liquid crystal display device but smaller than that of the transparent liquid crystal device (with backlighting). Just as in the case of the transparent liquid crystal display device, a display can be viewed clearly on the screen of the display device under the dark environment. Under the normal lighting environment, however, such a display cannot be viewed sufficiently. Therefore, the adaptability to different use environments is insufficient in case of either the upper output, system or the lower output system.
Furthermore, the reflective liquid crystal display device utilizes outside light from the environment as light for display. Therefore, principally, there is no need of backlighting on the display's side. In other words, power is only required for driving the liquid crystal and the driving circuit. Therefore, power saving can be positively attained. However, as opposed to the former display devices, a display can be clearly viewed under the lighting environment, but it cannot be clearly viewed under the dark environment. Considering the uses of the handheld terminals, they are mainly used outdoor and displays on their screens may be viewed under comparatively bright environments in most cases. In this case, however, the adaptability to different use environments is still insufficient. Therefore, front-lighting devices are installed in some of the commercially available handheld terminals such that they can be provided as reflective display devices capable of providing displays under the dark environments.
Consequently, attention is being given to a transflective liquid crystal display device because it is constructed as a combination of transparent and reflective liquid crystal display devices so as to have their advantageous features. Under the lighting environment the transflective liquid crystal display device utilizes the power-saving property and the good visibility of the reflective type, while under the dark environment, it utilizes the good-contrasting property of the transparent type using the backlight.
The transflective liquid crystal display device is disclosed in Japanese Patent Laid-Open No. Hei 11-101992. In this document, there is disclosed a dual-use type (transflective type) reflective transparent liquid crystal display device integrated with a reflective part by which outside light is reflected and a transparent part through which light from the backlighting is transmitted on a single display pixel. When the device is in total darkness, it is functioned as a dual-use type liquid crystal display device where a display is viewed on the screen using light emitted from the backlighting and passed through the transparent part and light reflected from the reflective part formed by a film having a comparatively high reflectivity. When the device is well lighted with outside light, it is functioned as a reflective type liquid crystal display device where a display is viewed on the screen using light reflected from the reflective part formed by a film having a comparatively high reflectivity.
Furthermore, in the transflective liquid crystal display device, a specific uneven structure (e.g., projections and depressions) having a light-diffusing property is provided particularly on a reflective part that allows a reflective display on the screen. In the case of the reflecting electrode, because of the structural design thereof, the light incident on the surface of the reflecting electrode at a certain incident angle in a certain direction is confined such that the light can be only reflected from the electrode at a specific output angle in a specific direction (Snell's law). Therefore, the angle and direction of the output light is specifically defined with respect to those of the incident light when the surface of the reflecting electrode is flat. Any display device being produced under such conditions shows a display with an extremely poor visibility.
In the transparent and reflective liquid crystal display devices, furthermore, the placement of color filter is one of causes of parallax and blurred image.
For example, in the case of a transparent liquid crystal display panel as shown in FIG. 16A, it generally includes a first substrate (an device substrate) 1601 having a first electrode (a transparent electrode) 1602 provided as a pixel electrode, a second substrate (a counter substrate) 1603 having a second electrode (a transparent electrode) 1606 provided as a counter electrode, and a liquid crystal layer 1607, wherein a black matrixes (BMs) (1) 1604 and color filters 1605 are formed between the second substrate (the counter substrate) 1603 and the second electrode (the transparent electrode) 1606.
For attaining a higher resolution, compared with one shown in FIG. 16A, an alternative configuration of the LCD panel may be provided as shown in FIG. 16B.
In the transparent LCD panel, the BMs are assembled typically for hiding an escape of light caused by irregularities in the orientation of liquid crystal at the time of driving the liquid crystal. In the case of forming the BMs and the color filters on the side of the counter substrate, in general, the dimensions of the BM are defined so as to be slightly larger than the predetermined dimensions thereof within predetermined ranges as margins on the assumption that the device substrate and the counter substrate would be deviated within a predetermined range during the step of combining these substrates together in the process of forming liquid-crystal display devices.
Therefore, in the high-precision panel as shown in FIG. 16B, an opening portion (an opening portion (2)) is sacrificed to ensure the above margin (BM margin (2)), so that it would lead to a serious decrease in numerical aperture.
Therefore, as a method for solving the problem of a decrease in numerical aperture accompanied by an increase in resolution, it is considered to form color filters 1623 on the first substrate (the device substrate) 1621 as shown in FIG. 16C.
In the case of showing in FIG. 16C, there is no need to provide the BMs (3) with margins against the error of lamination, so that an opening portion (3) can be obtained without sacrificing the numerical aperture.
On the contrary, as shown in FIG. 17A, the configuration of the reflective liquid crystal display device publicly known in the art includes a first substrate (a device substrate) 1701 having a first electrode (a reflective electrode) 1702 provided as a pixel electrode a second substrate (a counter substrate) 1703 having a second electrode (a transparent electrode) 1706 provided as a counter electrode, and a liquid crystal layer 1707, wherein black matrixes (BMs) (4) 1704 and color filters 1705 are formed between the second substrate (the counter substrate) 1703 and the second electrode (the transparent electrode) 1706. In this case, furthermore, the BMs (4) have the margins in consideration of an escape of light and the error of lamination between the first substrate (the device substrate) 1701 and the second substrate (the counter substrate) 1703, so that the dimensions of an opening portion can be restricted with the BM margins (4). In other words, the dimension of the opening portion can be represented as an opening portion (4) in the figure.
In this case, regarding the light (1) as shown in FIG. 17A, incident light and output light pass through the color filter formed on the same pixel. Regarding the light (2) and (3), on the other hand, incident light and output light pass through the color filters formed on the different pixels. In other words, the possibility of which the light passes through the color filters formed on the different pixels increases when the color filters are formed on the side of the counter substrate. In some cases, a problem of causing blurred images may arise.
Therefore, for solving such a problem of causing blurred images, as shown in FIG. 17B, a method of forming color filters 1714 on the first substrate (the device substrate) 1711 would be appropriate.
In FIG. 17B, there is illustrated a favorable, method for preventing the generation of blurred images. In this case, an opening portion (not shown) can be formed without sacrificing the numerical aperture because of no need to provide the BMs (5) with margins to the error of lamination. Also, the ratio of which the incident light and the output light pass through the color filter formed on the same pixel increases, compared with that of FIG. 17A.
In this case, however, there is another problem of a decrease in effective applied voltage because of having a laminated structure obtained by laminating the liquid crystal layer 1717 and the color filter 1714 together and the color filter 1714 is formed as part of the capacitance of the pixel.
It may be said that the transflective liquid crystal display device is one that copes with a special usage condition named a handheld terminal. In particular, it is expected that great demand is anticipated by the application to cellar phones in future. For ensuring stable demand or addressing enormous demand, it is clear that there is the need of increasing activity of cost reduction.
However, for forming an uneven structure as described above, there is the need of providing a method in which a reflecting electrode is mounted after forming an uneven structure on a layer to be located below the reflecting electrode. In this process, for realizing such a configuration, the number of steps increases because of the need of patterning for forming the uneven structure. The increase in the number of the steps will cause disadvantageous situations including a decrease in yielding percentage, an extension of processing time and cost increase.
Therefore, an object of the present invention is to provide a transflective liquid crystal display device having a reflective electrode with an uneven structure, which is formed without increasing the number of the steps in the process.
Furthermore, another object of the present invention is to provide a transflective liquid crystal display device having an excellent visibility by optimizing the arrangement of a color filter, which becomes controversial when the transparent or reflective liquid crystal display device is fabricated, for the transflective liquid crystal display device.